Base for honeycomb filter, method for producing same and honeycomb filter

ABSTRACT

A base  1  for a honeycomb filter of the present invention includes: a ceramic porous body having a large number of fine pores, and a plurality of cells  3  separated from each other by partition walls  4  and to serve as fluid passages. 50% pore diameter (d 50 ) of the ceramic porous body is within the range from 8.5 to 13 μm, and the partition walls  4  separating the plurality of cells have an average surface roughness of 3.0 to 5.5 μm.

TECHNICAL FIELD

The present invention relates to a honeycomb filter, a base for ahoneycomb filter, and a production method thereof. Particularly, thepresent invention relates to a honeycomb filter excellent inimpurity-removing performance and having high fluid-permeation amount(that is, treatment capacity), a base for a honeycomb filter suitablyusable for production of such a honeycomb filter, and a productionmethod thereof.

BACKGROUND ART

There has recently been used, in wide fields such as fields of watertreatment, medicine, food, etc., a filter using a ceramic porous body asa filtering material in order to remove impurities such as suspendedmatter, bacteria, and dust coexisting in fluid (liquid, gas).

As such a filter, there has widely been used, for example, a honeycombfilter 22, as shown in FIG. 2, constituted by a ceramic porous body witha large number of pores and having a plurality of cells 23 separatedfrom each other by partition walls and to serve as fluid passages(hereinbelow, such a shape is referred to as a “honeycomb shape”).

In such a honeycomb filter described above, when fluid to be treated issupplied into the plurality of cells, the fluid permeates through aceramic porous body constituting the honeycomb filter and flow out ofthe peripheral surface. At that time, suspended matter or the like isremoved. Therefore, purified fluid can be collected by housing thehoneycomb filter in a case in the state that the peripheral surface sideand the opening end face side is fluid-tightly isolated from each otherby a sealing material (e.g., O-ring).

Most honeycomb structures employ a structure having a base constitutedby a ceramic porous body having a large number of fine pores andseparated from each other by partition walls, and a filtration membraneconstituted by a porous body having pores having smaller average porediameter than those of the base and formed on a surface of the partitionwalls separating a plurality of cells from each other (e.g.,JP-A-2001-260117 and JP-A-2001-340718).

In the above structure, impurity-removing performance is secured bymaking the average pore diameter of the filtration membrane smaller thana particle diameter of impurities (about 0.01 to 1.0 μm), while it ispossible to lower the flow-resistance, thereby increasingfluid-permeation amount and improving treatment capacity by making theaverage pore diameter of the base larger than that of the filtrationmembrane (about 1 to 100 μm). That is, constitution of a honeycombfilter having high capacity can be achieved by making the average porediameter of the base as large as possible.

However, in the case of forming a filtration membrane having an averagepore diameter smaller than that of the base on a surface of partitionwalls separating a plurality of cells from each other in a base having alarge average pore diameter, there are the following problems.

That is, upon producing a honeycomb filter, it is general to form afiltration membrane by applying slurry containing aggregate particles ona surface of partition walls of a base (i.e., inner walls of cells) toobtain a film-forming body, which is then dried and fired. When afiltration membrane having a smaller average pore diameter is formed,slurry containing aggregate particles having a smaller average particlediameter is used. However, if slurry containing aggregate particleshaving a smaller average particle diameter is applied on a surface ofpartition walls of a base (i.e., inner walls of cells) having a largeaverage pore diameter, slurry does not stay only on a surface of thepartition walls of the base and permeates into the inside of the poresof the base to block up the pores of the base. Therefore, a problem isthat increase in fluid-permeation amount and improvement in treatmentcapacity cannot be achieved as much as expected.

As a method of avoiding the above problem, consideration has been givento a method in which an intermediate membrane of a porous body having anaverage pore diameter between those of the base and the filtrationmembrane is formed between the base and the filtration membrane.According to this method, since aggregate particles in slurry aretrapped on the surface of the intermediate membrane, aggregate particlesare inhibited from permeating into pores of the base. However, totalthickness of the filtration membrane and the intermediate membrane islarge, and thereby increasing flow-resistance of fluid in this portion.Therefore, even if this method is employed, it is impossible to increasefluid-permeation amount and improve treatment capacity of a filter.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of these conventionalproblems, and there is provided a honeycomb filter having advantageouseffects of large fluid-permeation amount and high treatment capacity. Tobe concrete, there is provided a base for a honeycomb filter, the basebeing suitably usable for production of such a honeycomb filter, and amethod for producing the base.

The present inventors earnestly studied to solve the above problems andfound that the aforementioned problems can be solved by specifying 50%pore diameter (d₅₀) of a ceramic porous body constituting a base for ahoneycomb filter to be within the range from 8.5 to 13 μnm andspecifying average surface roughness of partition walls separating aplurality of cells from each other to be within the range from 3.0 to5.5 μm. Thus, the present invention has been accomplished. That is,according to the present invention, there are provided the followingbase for a honeycomb filter, method for producing the base, andhoneycomb filter.

-   [1 ] A base for a honeycomb filter comprising:    -   a ceramic porous body having a number of fine pores, and    -   a plurality of cells separated from each other by partition        walls, the cells functioning as fluid passages;    -   wherein 50% pore diameter (d₅₀) of said ceramic porous body is        within the range from 8.5 to 13 μm, and the partition walls        separating the plurality of cells have an average surface        roughness of 3.0 to 5.5 μm,        where “50% pore diameter (d₅₀)” is a pore diameter measured by a        method of mercury penetration and calculated from a pressure        when a cumulative volume of mercury press-fitted into the porous        body is 50% of the volume of the whole pores of the porous body.-   [2] A method for producing a base for a honeycomb filter, comprising    the steps of:    -   mixing and kneading aggregate particles and water to obtain        clay,    -   forming the clay in a honeycomb shape having a plurality of        cells separated from each other by partition walls, the cells        functioning as fluid passages,    -   drying the clay in a honeycomb shape to obtain a honeycomb        formed body, and    -   firing the honeycomb formed body to obtain the base for the        honeycomb filter;    -   wherein 50% particle diameter (D₅₀) of the aggregate particles        is within the range of 50 to 70 μm, and the 50% particle        diameter (D₅₀) with 25% particle diameter (D₂₅) and 75% particle        diameter (D₇₅) satisfies the relation of the following        formulae (1) and (2):        0.4<D ₂₅ /D ₅₀  (1)        D ₇₅ /D ₅₀<1.4  (2)    -    where “x % particle diameter (D_(x))” is a particle diameter        measured by a sieving method, the particle diameter being at the        point where cumulative mass of powder meets x % of the whole        mass on a particle size distribution curve given from a relation        between a mesh diameter and mass of powder remaining on the        sieve.-   [3] A method for producing a base for a honeycomb filter according    to Claim 2, wherein the 50% particle diameter (D₅₀) of the aggregate    particles with thickness (W) of the partition walls of the base for    the honeycomb filter satisfies the following formula (3):    D ₅₀ W≦0.12  (3)-   [4] A honeycomb filter comprising:    -   a base for the honeycomb filter according to the above [1]    -   an intermediate membrane comprising a porous body having smaller        50% pore diameter (d₅₀) than the ceramic porous body        constituting said base, the intermediate membrane being formed        on a surface of the partition walls separating the plurality of        cells from each other of the base for the honeycomb filter, and    -   a filtration membrane comprising a porous body having smaller        50% pore diameter (d₅₀) than the porous body constituting the        intermediate membrane, the filtration membrane being formed on a        surface of the intermediate membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view schematically showing an embodiment of a base fora honeycomb filter and showing a structure viewed from a cell-openingend face side.

FIG. 2 is a perspective view schematically showing an embodiment of ahoneycomb filter.

BEST MODE FOR CARRYING OUT THE INVENTION

Upon developing a honeycomb filter of the present invention, the presentinventors made a study on a cause of increasing flow-resistance of fluidin a portion of intermediate membrane because of large total thicknessof a filtration membrane and an intermediate membrane in a method inwhich an intermediate membrane of a porous body having an average porediameter between those of the base and the filtration membrane is formedbetween the base and the filtration membrane. As a result, it was foundout that a thick intermediate membrane has conventionally been formed tomake a surface of the intermediate membrane as flat and smooth aspossible.

It is required to form a surface of an intermediate membrane, whichserves as a foundation layer for a filtration layer as flat and smoothas possible in order to inhibit generation of a film defect in thefiltration membrane. However, due to the large rough unevenness of asurface of partition walls of a conventional base, it was necessary tofill up the unevenness of partition walls in the first place uponforming an intermediate layer. Then, a surface of the intermediate layeris made flat and smooth. Therefore, the intermediate layer had to bethick.

Therefore, in the present invention, 50% pore diameter (d₅₀) of aceramic porous body constituting a base for a honeycomb filter isspecified to 8.5 to 13 μm, and average surface roughness of partitionwalls separating a plurality of cells from each other is specified to3.0 to 5.5 μm.

This makes a surface of partition walls of a base relatively smooth andflat to reduce unevenness. Therefore, upon forming an intermediatemembrane, filling up of unevenness of partition walls is not necessary,and the surface can be made flat and smooth even if the intermediatelayer is thin. Accordingly, with inhibiting generation of a defect of afiltration membrane, the total thickness of the filtration membrane andthe intermediate membrane can be reduced, which enables to reduceflow-resistance of fluid in this portion. That is, fluid-permeationamount is increased, and treatment capacity can be improved.

Hereinbelow will specifically be described embodiments of a base for ahoneycomb filter, a method for producing the base, and a honeycombfilter.

(1) Base for a Honeycomb Filter

A base for a honeycomb filter of the present invention is constituted bya ceramic porous body having a large number of pores and has a pluralityof cells 3 which are separated from each other by partition walls 4 andfunctions as fluid passages as, for example, the base 1 for a honeycombfilter shown in FIG. 1.

There is no particular limitation to shapes of the base for a honeycombfilter (hereinbelow sometimes referred to simply as “base”) as long asit is a honeycomb shape having a plurality of cells (through holes) tofunction as fluid passages as described above. Examples of the wholeshape include a cylindrical shape as shown in FIG. 1, a quadrangularprism, and a triangular prism. Examples of the cell shape (cell shape ina section perpendicular to a direction of cell formation) include arectangle as shown in FIG. 1, a circle, a hexagon, and a triangle.

A base is generally constituted by ceramic because ceramic has highreliability because of superior physical strength and durability incomparison with organic polymer, ceramic is hardly deteriorated even bywashing with acid or alkali because of high corrosion resistance, andthe average pore diameter which determines filtration ability can beaccurately controlled. There is no particular limitation to kinds ofceramic, and examples of ceramic include cordierite, mullite, alumina,powder of potsherd, aluminum titanate, lithium aluminum silicate,silicon carbide, silicon nitride, and a mixture thereof.

A base is constituted by a ceramic porous body having a large number ofpores. In the present invention, 50% pore diameter (d₅₀) is necessarilywithin the range from 8.5 to 13 μm. When 50% pore diameter (d₅₀) isbelow the above range, flow-resistance increases when fluid permeatesinside the base, which reduces fluid-permeation amount and lowersmechanical strength of the base, which is not preferable. When 50% porediameter (d₅₀) is above the above range, mechanical strength of the baseis lowered, which is not preferable.

Incidentally, “50% pore diameter (d₅₀)” in the present invention means apore diameter measured by a method of mercury penetration and calculatedfrom a pressure when a cumulative volume of mercury press-fitted intothe porous body is 50% of the capacity of the hole pores of the porousbody. A method of mercury penetration is a method for measuring a porediameter with the following formula (4) as a principle formula. To beconcrete, when mercury is press-fitted into a dried porous body withpressure being gradually raised, mercury is press-fitted in order frompores having larger diameters, and a cumulative volume of mercuryincreases. When all pores are filled with mercury finally, thecumulative volume reaches equilibrium volume (which corresponds to thewhole pore volume of the porous body). In the present invention, a porediameter d calculated from a pressure P when a cumulative volume ofmercury press-fitted into the porous body reached 50% of the capacity ofthe whole pores of the porous body was defined as “50% pore diameter(d₅₀)”. In this measurement method, “50% pore diameter (d₅₀)” is aso-called average pore diameter.d=−yγcos θ/P  (4)(where d represents pore diameter, γ represents surface tension atliquid-air interface, θ represents contact angle, and P representspressure) In a base for a honeycomb filter of the present invention, itis necessary that partition walls separating a plurality of cells fromeach other have an average surface roughness within the range from 3.0to 5.5 μm. When the average surface roughness is below the above range,a surface of partition walls is extremely flat and smooth, and anintermediate layer is prone to exfoliate from the surface of partitionwalls when the intermediate layer is formed on a surface of partitionwalls. On the other hand, when the average surface roughness is abovethe range, the partition walls have coarse surface having largeunevenness. Therefore, when an intermediate is formed, it is necessaryto employ a thick intermediate membrane in order to make a surface ofthe intermediate membrane flat and smooth besides filling up of theunevenness of the partition walls in the first place. That is, sinceflow-resistance of fluid increases in the intermediate membrane portion,fluid-permeation amount of a resultant honeycomb filter decreases, andtreatment capacity is lowered.

Incidentally, “surface roughness” in the present invention means surfaceroughness measured according to JIS B0601 “Surface Roughness—Definitionand Indication”. To be concrete, a standard length of a roughness curvewas taken out in the direction of an average line, a surface roughnesscurve having the standard length was folded back based on the averageline, and a value obtained by dividing an area surrounded by the surfaceroughness curve and the average line by the standard length wasindicated by micrometer (μm), which was defined as surface roughness(Ra). In addition, “average surface roughness” of the present inventionmeans an average value obtained by measuring the aforementioned surfaceroughness (Ra) at 10 places arbitrarily selected on the surface ofpartition walls separating a plurality of cells from each other of abase for a honeycomb filter.

(2) Method for Producing a Base for a Honeycomb Filter

In a method for producing a base for a honeycomb filter of the presentinvention, at least aggregate particles and water are mixed and kneadedto obtain clay, which is formed in a honeycomb shape having a pluralityof cells functioning as fluid passages and dried to obtain a honeycombformed body, which is fired to obtain a base for a honeycomb filter.

The aggregate particles are particles serving as the main componentconstituting the base (sintered body). There is no particular limitationto kinds of the aggregate particles, and examples of the aggregateparticles include cordierite, mullite, alumina, powder of potsherd,aluminum titanate, lithium aluminum silicate, silicon carbide, siliconnitride, and a mixture thereof.

In the production method of the present invention, it is necessary touse aggregate particles having 50% particle diameter (D₅₀) in the rangefrom 50 to 70 μm. When 50% particle diameter (D₅₀) is below the range,since a resultant base has small pore diameters, flow-resistance offluid is increased when fluid permeates through the inside of the base,which is not preferable in that fluid-permeation amount is reduced andtreatment capacity is lowered.

On the other hand, when 50% particle diameter (D₅₀) is above the range,since a resultant base has large pore diameters, aggregate particles inslurry for forming film enter the pores or pass through the pores of thebase, which is not preferable in that it causes increase in film-forminginferiorities such as a film defect. In addition, in the case that amethod in which clay containing aggregate particles is extruded using anextrusion die having a shape complementary to a desired honeycombstructure (cell shape, thickness of partition walls, cell density, etc.)is employed as a method for forming a honeycomb formed body, clogging isprone to be caused in the portion corresponding to partition walls of abase (slit portion of the extrusion die) in the extrusion die.Therefore, an extruded honeycomb formed body has many defects, which isnot preferable in that a yield of a honeycomb formed body is lowered.

In a production method of the present invention, besides the aggregateparticles satisfying the above conditions, it is preferable that 50%particle diameter (D50) of the aggregate particles with thickness (W) ofpartition walls of the base for a honeycomb filter satisfies thefollowing formula (3):D ₅₀ /W≦0.12  (3)

By satisfying the relation of the above formula (3), it is possible toeffectively inhibit deterioration in yield of a honeycomb formed bodydue to clogging in a slit portion of the above extrusion die.Incidentally, “partition walls” in the present invention mean the wholeportion separating a plurality of cells from each other in a base andare not limited to ones in which portions separating a plurality ofcells from each other have a uniform thickness. For example, when a basehas a circle cell shape, a portion separating a plurality of cells fromeach other does not have a uniform thickness. However, such a portion isalso included in “partition walls” of the present invention.Incidentally, when a portion separating a plurality of cells from eachother does not have a uniform thickness, a definition of the above“thickness (W) of partition walls” seems a problem. However, thicknessof the thinnest portion among the portions separating a plurality ofcells from each other is defined as “thickness (W) of partition walls”.

A characteristic of a production method of the present invention is theuse of aggregate particles having a broader particle distribution incomparison with a conventional one. Since such aggregate particlescontain a relatively large amount of particles having small particlediameters, partition walls of a base can have small average surfaceroughness. In such a method, pore diameter distribution of a base isalso broad. However, unlike a filtration membrane, which is required tosecure a function of securely removing impurities in fluid by having apredetermined pore diameter, a base is not required to have a sharp porediameter distribution as long as it has small flow-resistance uponpermeation of fluid inside the base, large fluid-permeation amount, andhigh treatment capacity. Therefore, it can be considered that it is notnecessary to use aggregate particles having a sharp particle sizedistribution for the purpose of sharpening a pore diameter distributionof a base as a conventional production method. To be concrete, there isused aggregate particles in which 50% particle diameter (D₅₀) with 25%particle diameter (D₂₅) and 75% particle diameter (D₇₅) satisfies therelation of the following formulae (1) and (2):0.4≦D ₂₅ /D ₅₀  (1)D ₇₅ /D ₅₀≦1.4  (2)

When the above formula (1) is not satisfied, percentage of aggregateparticles having small particle diameters is too high, and it isapprehended that a resultant base has small pore diameters. That is, ina base to be produced, flow-resistance is large when fluid passes insidethe base, which is not preferable in that fluid-permeation amountdecreases and treatment capacity is lowered. On the other hand, when theabove formula (2) is not satisfied, it is not preferable in that it isapprehended that a yield of a honeycomb formed body is lowered due tothe aforementioned clogging in a slit portion of an extrusion die.

Incidentally, “x % particle diameter (D_(x))” of the present inventionmeans a particle diameter measured by a sieving method, i.e., a particlediameter at the point where cumulative mass of powder meets x % of thewhole mass on a particle size distribution curve given from a relationbetween a mesh diameter and mass of powder remaining on the sieve usinga plurality of sieves having different normal mesh diameters. To beconcrete, a plurality of sieves having different normal mesh diametersare piled up so that one having a larger mesh diameter locates upper, apowder sample to be measured for particle diameter was put in theuppermost sieve and shaken for 15 minutes with a shaker, a particle sizedistribution curve was prepared from a relation between mass of powderremaining on each sieve and a mesh diameter of the sieve, and a particlediameter at the point where cumulative mass of powder meets x % of thewhole mass was defined as x % particle diameter (D_(x)). In this methodfor measurement, “50% particle diameter (D₅₀)” is a so-called averageparticle diameter.

Examples of the method for preparing aggregate particles having theaforementioned 50% particle diameter (D₅₀) and particle sizedistribution include a method using a commercial ceramic raw materialwhich is left as it is or pulverized and classified and a method inwhich two or more kinds of such aggregate particles are suitably mixedso as to satisfy the aforementioned conditions.

As a production method of the present invention, there may be employedthe same method as a conventional method for producing a base for ahoneycomb filter except for the use of the aggregate particles asdescribed above. In the first place, at least aggregate particles asdescribed above and water are mixed and kneaded to give clay.

Incidentally, the clay may contain other additives, e.g., an organicbinder, a dispersant, and an inorganic bonding material, as necessarybesides the aggregate particles and water.

An organic binder is an additive which turns into gel in a formed body(clay) before firing and functions as a reinforcer for maintainingmechanical strength of the formed body. Therefore, there can suitably beused as the organic binder an organic polymer gellable in a formed body(clay), for example, hydroxypropylmethyl cellulose, methyl cellulose,hydroxyethyl cellulose, carboxylmethyl cellulose, and poly(vinylalcohol).

A dispersant is an additive for facilitating dispersion of aggregateparticles in water as a dispersion medium. Examples of the dispersantinclude ethylene glycol, dextrin, fatty acid soap, and polyalcohol.

An inorganic bonding material is an additive for strengthening the bondamong aggregate particles, and there may be used at least one kindselected from the group consisting of alumina, silica, zirconia,titania, glass frit, feldspar, and cordierite, having an averageparticle diameter of 10 μm or less. Though the inorganic bondingmaterial is a ceramic particle, it is not included in “aggregateparticles” of the present invention.

It is preferable to add 10 to 35 parts by mass of the inorganic bondingmaterial with respect to 100 parts by mass of the aggregate particles.When it is below 10 parts by mass, it is not preferable in that strengthof the base is lowered. When it is above 35 parts by mass, thoughstrength is enhanced, the organic bonding material stays in a gap amongthe aggregate particles, and thereby pores inside the base are cloggedto lower the fluid-permeation amount, which is not preferable.

Clay having appropriate viscosity can be prepared by mixing the aboveaggregate particles, water, organic binder, etc., and kneading themixture with, for example, a vacuum pug mill. The clay is formed in ahoneycomb shape and dried to obtain a honeycomb formed body.

As a forming method, a conventionally known forming method such asextrusion, injection, or press molding may be employed. Among them,there may suitably be employed a method of subjecting the clay preparedas described above to extrusion using an extrusion die having a shapecomplementary to a desired honeycomb structure (cell shape, thickness ofpartition walls, cell density, etc.). As a drying method, aconventionally know drying method such as hot air drying, microwavedrying, dielectric drying, drying under reduced pressure, vacuum drying,or freeze drying may be employed. Among them, it is preferable to employa method of hot air drying in combination with microwave drying ordielectric drying in that the whole honeycomb formed body can be driedrapidly and uniformly.

Finally, the honeycomb structure obtained as described above is fired toobtain a base for a honeycomb filter. Firing is an operation forsecuring a predetermined strength by sintering aggregate particles inthe honeycomb formed body to densify the honeycomb formed body. Suitablefiring conditions (temperature, time) may be selected according to kindof aggregate particles to be used. For example, when silicon carbide isused for aggregate particles, it is preferable to fire at 1300 to 2300°C. for about one to five hours.

Incidentally, it is preferable to remove organic matter (organic binder,etc.) in the honeycomb formed body by combustion (calcination) beforefiring or in the temperature-rising process in that removal of organicmatter can be facilitated. Though the calcination time is notparticularly limited, it is generally about one to ten hours.

(3) Honeycomb Filter

A honeycomb filter of the present invention includes the aforementionedbase for a honeycomb filter, an intermediate membrane formed on asurface of partition walls separating a plurality of cells of the basefor a honeycomb filter and constituted by a porous body having smaller50% pore diameter (d₅₀) than a ceramic porous body constituting thebase, and a filtration membrane formed on a surface of the intermediatemembrane and constituted by a porous body having smaller 50% porediameter (d₅₀) than a porous body constituting the intermediatemembrane. It is not necessary in such a honeycomb filter to thicken theintermediate layer because the aforementioned special structure of thebase for a honeycomb filter inhibits generation of defects in afiltration membrane. Therefore, flow-resistance of fluid can be reducedin a portion of the intermediate membrane, and it is possible to enhancetreatment capacity by increasing fluid-permeation amount of the filter.

A honeycomb filter of the present invention can be produced by formingan intermediate layer on a surface of partition walls in a base for ahoneycomb filter by a conventionally known film-forming method andfurther forming a filtration membrane on a surface of the intermediatelayer. For example, a film-forming slurry containing at least aggregateparticles and water (further, an organic binder, a pH-adjusting agent, asurfactant, etc.) is applied on a surface of partition walls of theaforementioned base for a honeycomb filter to obtain a film-formingbody, which is then dried and fired to form an intermediate membrane anda filtration membrane.

In addition, film-forming slurry may contain an inorganic bondingmaterial for the same purpose as the case of producing a base. However,as an inorganic bonding material contained in a film-forming slurry,clay mineral, kaolin, titania sol, silica sol, glass frit, or the like,having an average diameter of 1 μm or less can be used unlike aninorganic bonding material contained in clay for formation, and it ispreferable to add 5 to 20 parts by mass of an inorganic bonding materialwith respect to 100 parts by mass of aggregate particles from theviewpoint of securing film strength.

There is no particular limitation to kind of a film-forming method, andthere may be employed, for example, a dipping film-forming method, afiltration membrane-forming method described in JP-B-63-66566. There maybe used the same aggregate particles, organic binder, and the like, asthose used in production of the base. However, since it is necessary tomake 50% pore diameter (D₅₀) smaller in the order of the base, theintermediate membrane, and the filtration membrane, 50% pore diameter(D₅₀) of aggregate particles is generally made smaller in the order ofthe base, the intermediate membrane, and the filtration membrane.

EXAMPLES

Hereinbelow will be described a base for a honeycomb filter, a methodfor producing the base, and a honeycomb filter of the present inventionmore specifically with referring to Examples. However, a base for ahoneycomb filter, a method for producing the base, and a honeycombfilter of the present invention are by no means limited to theseExamples.

(Method for Measuring Physical Properties and Method for Evaluation)[25% Particle Diameter (D₂₅), 50% Particle Diameter (D₅₀), 75% ParticleDiameter (D₇₅)]

A plurality of sieves having different normal mesh diameters are piledup so that one having a larger mesh diameter locates upper, a powdersample to be measured for particle diameter was put in the uppermostsieve and shaken for 15 minutes with a shaker, a particle sizedistribution curve was prepared from a relation between mass of powderremaining on each sieve and a mesh diameter of the sieve, and particlediameters at the point where cumulative mass of powder meets 25%, 50%,and 75% of the whole mass were defined as 25% particle diameter (D₂₅),50% particle diameter (D₅₀), and 75% particle diameter (D₇₅),respectively. Incidentally, when an “average particle diameter” issimply referred to in the following Examples or Comparative Examples, itmeans “50% particle diameter (D₅₀)”.

[Yield of Formed Body]

A yield of a formed body was defined from a percentage of the number ofhoneycomb formed bodies without any defect due to clogging in a slitportion of an extrusion die (i.e., passed article) with respect to 100honeycomb formed bodies produced. Evaluations were made as good when theyield was above 90%, fair when it was 90% or less, and bad when it was80% or less.

[Average Surface Roughness]

Average surface roughness was calculated from surface roughness (Ra)measured according to JIS B0601 “Surface Roughness—Definition andIndication”. A standard length of a roughness curve was taken out in thedirection of an average line, a surface roughness curve having thestandard length was folded back based on the average line, and a valueobtained by dividing an area surrounded by the surface roughness curveand the average line by the standard length was indicated by micrometer(μm), which was defined as surface roughness (Ra). The average valueobtained by measuring the aforementioned surface roughness (Ra) at 10places arbitrarily selected on the surface of partition walls separatinga plurality of cells from each other of a base for a honeycomb filterwas defined as the “average surface roughness”.

[50% Pore Diameter (d₅₀)]

50% pore diameter (d₅₀) was measured by a method of mercury penetration.A sample having a predetermined shape was cut out from a base for ahoneycomb filter or a honeycomb filter in Examples or ComparativeExamples. Mercury was press-fitted into the sample with pressure beinggradually raised, and a pore diameter d calculated based on thefollowing formula (4) from a pressure P when a cumulative volume ofmercury press-fitted into the porous body reached 50% of the volume ofthe whole pores of the porous body was defined as “50% pore diameter(d₅₀)”.d=−γ×cos θ/P  (4)(where d represents pore diameter, γ represents surface tension atliquid-air interface, θ represents contact angle, and P representspressure)[Maximum Pore Diameter (d_(max)), Impurity-Removing Performance]

The maximum pore diameter (d_(max)) of a filtration membrane wasmeasured according to an air flow method described in ASTM F316. Ahoneycomb filter of Examples or Comparative Examples is wetted by waterhaving a temperature of 20° C., pressurized air was sent in with thepressure being gradually raised from inside a plurality of cells of thewetted honeycomb filter, a pore diameter d calculated based on the aboveformula (4) from the pressure P when an air bubble was observed in thefirst place on a peripheral surface of the honeycomb filter was definedas the maximum pore diameter (d_(max)). When the maximum pore diameter(d_(max)) was below 1.8 μm, no film defect was present, and evaluationwas given as a filter having excellent impurity-removing performance.When the maximum pore diameter (d_(max)) was 1.8 μm or more, a filmdefect was present, and evaluation was given as a filter havinginsufficient impurity-removing performance.

[Average Film Thickness]

Average thickness of an intermediate membrane and a filtration membranewas calculated from film thickness measured by a measuring microscope. Ahoneycomb filter of Examples or Comparative Examples was cut at a faceparallel to a cell opening end face. Along a direction of a diameter ofthe honeycomb filter, a film thickness of one row (44 cells) wasmeasured, and an average of the measured values was defined as anaverage film thickness.

[Water-Permeation Aamount, Fluid-Permeation Amount (Treatment Capacity)]

A honeycomb filter of Examples or Comparative Examples was left in waterfor two hours under a reduced pressure of 6.7 kPa or less to remove airbubbles in the honeycomb filter. Then, pure water was sent inside aplurality of cells of the honeycomb filter and penetrated from insidethe cells to the peripheral surface side of the honeycomb filter.Water-permeation amount per unit filtration area and unit hour wasmeasured. When the water-permeation amount was 1.67 m³/hr·m² or more,evaluation was given as a filter having large fluid-permeation amountand high treatment capacity. When the water-permeation amount was below1.67 m³/hr·m², evaluation was given as a filter having smallfluid-permeation amount and insufficient treatment capacity.

EXAMPLE, COMPARATIVE EXAMPLE

[Base for a Honeycomb Filter and Production Method]

First, there were prepared aggregate particles shown in Table 1, glassfrit having an average particle diameter of 3.5 μm as an inorganicbonding material, methyl cellulose as an organic binder, andpolyethylene glycol as a dispersant. Then, the aggregate particles, theinorganic bonding material, water, the organic binder, and thedispersant were mixed at a mass ratio of 100:11.1:13.1:3.6:0.9 to give amixture. The mixture was kneaded with a vacuum pug mill to obtain clayhaving adequate viscosity. TABLE 1 Base Formed body Average Aggregateparticles (base material) Clogging surface D₅₀ D₂₅ D₇₅ upon Yield d₅₀roughness Material (μm) (μm) D₂₅/D₅₀ (μm) D₇₅/D₅₀ D₅₀/W forming (%) (μm)(μm) Comp. Ex. 1 Alumina 45 15 0.3 83 1.8 0.07 Present 87 — — Comp. Ex.2 Alumina 45 18 0.4 65 1.4 0.07 None 99 6.7 2.8 Comp. Ex. 3 Alumina 4522 0.5 60 1.3 0.07 None 98 7.1 2.9 Comp. Ex. 4 Alumina 50 14 0.3 87 1.70.08 Present 82 — — Example 1 Alumina 50 18 0.4 72 1.4 0.08 None 99 8.53.2 Example 2 Alumina 50 22 0.4 68 1.4 0.08 None 98 8.8 3.3 Comp. Ex. 5Alumina 60 20 0.3 96 1.6 0.09 Present 85 — — Comp. Ex. 6 Alumina 60 240.4 92 1.5 0.09 Present 88 — — Example 3 Alumina 60 28 0.5 85 1.4 0.09None 100 10.4 4.1 Example 4 Alumina 60 35 0.6 84 1.4 0.09 None 100 11.24.5 Comp. Ex. 7 Alumina 70 25 0.4 110 1.6 0.11 Present 84 — — Example 5Alumina 70 30 0.4 101 1.4 0.11 None 98 12.1 5.0 Example 6 Alumina 70 380.5 99 1.4 0.11 None 100 12.4 5.1 Example 7 Alumina 70 43 0.6 95 1.40.11 None 99 12.6 5.1 Comp. Ex. 8 Alumina 75 25 0.3 113 1.5 0.12 Present81 — — Comp. Ex. 9 Alumina 75 30 0.4 105 1.4 0.12 None 98 14.2 5.8 Comp.Ex. 10 Alumina 75 30 0.4 105 1.4 0.12 None 100 14.2 5.8 Comp. Ex. 11Alumina 75 36 0.5 97 1.3 0.12 None 98 14.6 5.9 Comp. Ex. 12 Alumina 7536 0.5 97 1.3 0.12 None 100 14.6 5.9 Comp. Ex. 13 Alumina 85 32 0.4 1121.4 0.13 Present 75 — — Comp. Ex. 14 Alumina 90 36 0.4 126 1.4 0.14Present 70 — —

The above clay was subjected to extrusion in a honeycomb shape having aplurality of cells separated from each other by partition walls and tofunction as passages for fluid with a conventionally known extruderhaving an extrusion die having a shape complementary to a desiredhoneycomb structure (the whole shape, cell shape, and thickness ofpartition walls) and dried with hot air at 100° C. for 48 hours toobtain a honeycomb formed body. The honeycomb formed body was fired at1300° C. for two hours in an electric furnace to obtain a base for ahoneycomb filter (hereinbelow referred to simply as “base”).

The whole shape of the base obtained as above is cylindrical having acircle end face (cell opening face) having an outer diameter of 180 mmand a length of 1000 mm, with a cell shape of a hexagon whose inscribedcircle has a diameter of 2.5 mm, a thickness (W) of partition walls of650 μm, and the total number of 2000 cells. With regard to these bases,average surface roughness of partition walls is evaluated, and theresults are shown in Table 1.

[Results]

As shown in Table 1, with regard to Examples 1 to 7 where 50% particlediameter (D₅₀), D₂₅/D₅₀, and D₇₅/D₅₀ of aggregate particles which is araw material for the base was specified to be within the range of aproduction method of the present invention, bases each having 50% porediameter (d₅₀) of 8.5 to 13 μm and an average surface roughness ofpartition walls of 3.0 to 5.5 μm, and good results were shown. Inaddition, a yield of a honeycomb formed body greatly exceeded 90%. Thus,no problem was caused.

In Comparison Example 1 to 3, where 50% particle diameter (D₅₀) ofaggregate particles is below the range of a production method of thepresent invention, a base obtained had 50% pore diameter (d₅₀) of below8.5 μm, and it was expected that flow-resistance is large when fluidpass through the inside of the base. That is, it was expected that thefinally obtained honeycomb filter would have a reduced fluid-permeationamount and lowered treatment capacity.

In Comparison Example 8 to 14, where 50% particle diameter (D₅₀) ofaggregate particles is above the range of a production method of thepresent invention, a base obtained had 50% pore diameter (d₅₀) of above13 μm, and it was expected that film-forming inferiorities such as filmdefects would increase when an intermediate membrane or a filtrationmembrane is formed. Among them, clogging was caused in a slit portion ofan extrusion die upon extrusion forming of a base in Comparative Example8. Therefore, an extruded honeycomb formed body had many defects, and ayield of the honeycomb formed body was lowered to 90% or less. Further,with regard to Comparative Examples 13 and 14, where a value of 50%particle diameter (D₅₀) /thickness (W) of partition walls exceeded therange of a production method of the present invention, a yield of ahoneycomb formed body was as remarkably low as 80% or less. Further, inComparative Examples 1, 4 to 7, where a value of D₇₅/D₅₀ exceeds therange of a production method of the present invention, clogging wascaused in a slit portion of an extrusion die when a base is subjected toextrusion forming. Therefore, an extruded honeycomb formed body had manydefects, and a yield of the honeycomb formed body was lowered to 90% orless.

[Honeycomb Filter]

On the above base were formed an intermediate membrane and a filtrationmembrane by the following method to obtain a honeycomb filter.

First, there were prepared alumina particles having an average particlediameter of 3.2 μm as aggregate particles, glass frit having an averagediameter of 0.9 μm as an inorganic bonding material, methyl cellulose asan organic binder, and polycarboxylate as a dispersant. Then, theaggregate particles, inorganic bonding materials, water, organic binder,and dispersant were mixed at a mass ratio of 100:20:400:0.5:2.0 toprepare slurry for film formation (for an intermediate membrane).

There were also prepared alumina particles of an average particlediameter of 0.4 μm as aggregate particles, methyl cellulose as anorganic binder, and polycarboxylate as a dispersant. Then, the aggregateparticles, water, organic binder, and dispersant were mixed at a massratio of 100:1000:4.0:0.2 to prepare slurry for film formation (for afiltration membrane).

Then, the above slurry for film formation (for an intermediate membrane)was applied on a surface of partition walls of the above base using afiltration membrane-forming method described in JP-B-63-66566 to obtaina film-formed body, which was subjected to hot air drying at 100° C. fortwo hours and then fired at 1350° C. for two hours in an electricfurnace to form an intermediate membrane.

Further, the above slurry for film formation (for a filtration membrane)was applied on a surface of the intermediate membrane formed onpartition walls of the above base using a filtration membrane-formingmethod described in JP-B-63-66566 to obtain a film-formed body, whichwas subjected to hot air drying at 100° C. for 24 hours and then firedat 1300° C. for two hours in an electric furnace to form a filtrationmembrane. Thus, a honeycomb filter (hereinbelow referred to simply as“filter”) was obtained.

The filter obtained as described above had an intermediate membrane anda filtration membrane, each having an average thickness and 50% porediameter (d₅₀) shown in Table 2. Evaluation was made for the maximumpore diameter (d_(max)) and a water-permeation amount of these filters.The results are shown in Table 2. TABLE 2 Base Intermediate membraneFiltration membrane Filter Average Average Average Water- surface filmfilm permeation d₅₀ roughness thickness d₅₀ thickness d₅₀ d_(max) amount(μm) (μm) (μm) (μm) (μm) (μm) (μm) (m³/hr · m²) Comp. Ex. 1 — — — — — —— — Comp. Ex. 2 6.7 2.8 122 3.6 12 0.7 <1.8 1.61 Comp. Ex. 3 7.1 2.9 1203.6 10 0.7 <1.8 1.62 Comp. Ex. 4 — — — — — — — — Example 1 8.5 3.2 1193.6 11 0.7 <1.8 1.82 Example 2 8.8 3.3 121 3.6 10 0.7 <1.8 1.89 Comp.Ex. 5 — — — — — — — — Comp. Ex. 6 — — — — — — — — Example 3 10.4 4.1 1223.6 11 0.7 <1.8 2.04 Example 4 11.2 4.5 120 3.6 10 0.7 <1.8 2.15 Comp.Ex 7 — — — — — — — — Example 5 12.1 5.0 121 3.6 11 0.7 <1.8 2.26 Example6 12.4 5.1 199 3.6 10 0.7 <1.8 2.46 Example 7 12.6 5.1 120 3.6 11 0.7<1.8 2.49 Comp. Ex. 8 — — — — — — — — Comp. Ex. 9 14.2 5.8 122 3.6 100.7 5.4 2.51 Comp. Ex. 10 14.2 5.8 160 3.6 11 0.7 <1.8 1.51 Comp. Ex. 1114.6 5.9 121 3.6 11 0.7 6.7 2.54 Comp. Ex. 12 14.6 5.9 164 3.6 12 0.7<1.8 1.46 Comp. Ex. 13 — — — — — — — — Comp. Ex. 14 — — — — — — — —(Results)

As shown in Table 2, each of the filters in Examples 1 to 7 using a basehaving 50% pore diameter (d₅₀) and an average surface roughness ofpartition walls within the range of the present invention had awater-permeation amount of 1.67 m³/hr·m² or more and the maximum porediameter of a filtration membrane below 1.8 μm. Thus, good results wereshown with regard to removability of impurities and fluid-permeationamount (i.e., treatment capacity).

In addition, since each of the filters in Comparative Examples 2 and 3using a base having 50% pore diameter (d₅₀) below the range of thepresent invention has large flow-resistance when fluid permeates insidethe base, the water-permeation amount was below 1.67 m³/hr·m². That is,fluid-permeation amount was reduced, and treatment capacity was lowered.

Further, each of the filters in Comparative Examples 9 to 12 using abase having an average surface roughness of partition walls exceedingthe range of the present invention had a base having partition wallswhose surface was course and has large unevenness, when an intermediatemembrane is made thin as the filters in Comparative Examples 9 and 11, afilm defect was caused in a filtration membrane. That is, the maximumpore diameter (d_(max)) of the filtration membrane became 1.8 μm ormore, and removability of impurities was insufficient. In order to avoidsuch a situation, it is necessary to make the intermediate layer thickas the filters in Comparative Examples 10 and 12. However, such a filterhas large flow-resistance of fluid in the intermediate membrane portion.That is, each of the filters in Comparative Examples 10 and 12 had awater-permeation amount below 1.67 m³/hr·m², i.e., a smallfluid-permeation amount and lowered treatment capacity.

INDUSTRIAL APPLICABILITY

As described above, Since a base for a honeycomb filter of the presentinvention has a relatively smooth and flat surface of partition wallsand small unevenness, upon forming an intermediate membrane, filling upof unevenness of partition walls is not necessary, and even if theintermediate layer is thin, the surface can be made flat and smooth.This means that the total thickness of the filtration membrane and theintermediate membrane can be reduced with inhibiting generation of adefect of a filtration membrane, which enables to reduce flow-resistanceof fluid in this portion. That is, a base for a honeycomb filter of theinvention can suitably be used for producing a honeycomb filter havingexcellent removability of impurities, large fluid-permeation amount, andhigh treatment capacity.

1. A base for a honeycomb filter comprising: a ceramic porous bodyhaving a number of fine pores, and a plurality of cells separated fromeach other by partition walls, the cells functioning as fluid passages;wherein 50% pore diameter (d₅₀) of said ceramic porous body is withinthe range from 8.5 to 13 μm, and the partition walls separating theplurality of cells have an average surface roughness of 3.0 to 5.5 μm,where “50% pore diameter (d₅₀)” is a pore diameter measured by a methodof mercury penetration and calculated from a pressure when a cumulativevolume of mercury press-fitted into the porous body is 50% of the volumeof the whole pores of the porous body.
 2. (canceled)
 3. (canceled) 4.(canceled)
 5. A method for producing a base for a honeycomb filter,comprising the steps of: mixing and kneading aggregate particles andwater to obtain clay, forming the clay in a honeycomb shape having aplurality of cells separated from each other by partition walls, thecells functioning as fluid passages, drying the clay in a honeycombshape to obtain a honeycomb formed body, and firing the honeycomb formedbody to obtain the base for the honeycomb filter; wherein 50% particlediameter (D₅₀) of the aggregate particles is within the range of 50 to70 μm, and the 50% particle diameter (D₅₀) with 25% particle diameter(D₂₅), 75% particle diameter (D₇₅) and thickness (W) of the partitionwalls satisfies the relation of the following formulae (1) to (3):0.4≦D ₂₅ /D ₅₀  (1)D ₇₅ /D ₅₀≦1.4  (2)D ₅₀ /W≦0.12  (3) where “x % particle diameter (D_(x))” is a particlediameter measured by a sieving method, the particle diameter being atthe point where cumulative mass of powder meets x % of the whole mass ona particle size distribution curve given from a relation between a meshdiameter and mass of powder remaining on the sieve using a plurality ofsieves having different normal mesh diameters.
 6. A honeycomb filtercomprising: a base for the honeycomb filter including a ceramic porousbody having a number of fine pores, and a plurality of cells separatedfrom each other by partition walls, the cells functioning as fluidpassages; an intermediate membrane comprising a porous body havingsmaller 50% pore diameter (d₅₀) than the ceramic porous bodyconstituting said base, the intermediate membrane being formed on asurface of the partition walls separating the plurality of cells fromeach other of the base for the honeycomb filter, and a filtrationmembrane comprising a porous body having smaller 50% pore diameter (d₅₀)than the porous body constituting the intermediate membrane, thefiltration membrane being formed on a surface of the intermediatemembrane, wherein 50% pore diameter (d₅₀) of said ceramic porous body iswithin the range from 8.5 to 13 μm, and the partition walls separatingthe plurality of cells have an average surface roughness of 3.0 to 5.5μm, where “50% pore diameter (d₅₀)” is a pore diameter measured by amethod of mercury penetration and calculated from a pressure when acumulative volume of mercury press-fitted into the porous body is 50% ofthe volume of the whole pores of the porous body.